The challenge for an Engineer is what to do when you are rushed in melee. You do not have Immunity or Assault like a Soldier, so you definitely want to get clear enough to start shooting again. Damping is useless for stopping power despite a 3-second stun.See our discussion on the Soldier class and why Lift is the best choice for Shepard's Bonus Talent to handle melee crises. However, this means you can't get Sniper Rifles, so you'll have to make do with your pistol for long-range engagements. Which actually isn't that bad as a Pistol has good accuracy, so you can do quite a lot of things that a Sniper Rifle can, and without having to zoom in. The minimum spread means you don't have the same precision to do very cheap shots like shooting someone in the foot, but it's better than nothing and in the general case, you can get away with just a pistol and no fire support from your team.

If you can handle melee (i.e., with biotic teammates) and do want Sniper Rifles as your bonus talent, don't play an Engineer: Play an Infiltrator-Operative with Hacking as the bonus talent instead. They have slightly better class bonuses (damage and weapon cooldown): In comparison, the Engineer has 14% faster tech cooldown, but this is negligible after stacking on various other tech cooldown bonuses. They have stronger tech protection, but tech attacks don't happen that often, and in any case completely neutralizing an opponent with Damping or a biotic like Throw or Lift is better than toughing out an attack.

There is really only one choice for a Specialization since First Aid and Medicine are very weak Talents (see our discussion on Talents) so there is no point in being a Medic. Even if you did want a Medic, their bonuses are poor: Recharge time on Medical Talents only, and reviving party members can be done better with the Spectre training Unity talent, which requires no medi-gel.

Tali

Tali has frighteningly good shields because on top of Electronics, she has further shield bonuses from being a Quarian Machinist. Even so, because the combat AI for teammates isn't that great, try not to rely on her in combat. Instead, get her tech skills up first and keep her in the back while you take point and spam tech skills -- remember that when you use your teammates' tech skills, they deploy as if you deployed them, and they do not need to have line of sight to the target location.

With her remaining points, there are basically two options:

Improve Pistols and aim for higher Marksman skills: Do this only if you have Tali for mid to long range fire support.

Maximize Shotguns: Only if you know what you are doing with squads in close combat. Otherwise, Tali is too vulnerable in melee as she has no melee-crisis powers like Throw, Lift, or Immunity. Carnage sounds nice to have, but having a properly located squad member to shoot one on demand exactly when you need it is tricky at best.

Get some First Aid. But if you are not kamikaze in combat, you will hardly ever use medi-gel.